So…after sewing my own tarp and completing a few minor customizations, I figured that the next inevitable step down the MYOG rabbit hole is to try my hand at a pack. I have studied up on the tutorials from Dave Chenault and the Kringle pack, but I have a few questions before I finalize the design.

I am starting with a basic UL pack for my 4' 8" 80 lb 9 year old and would appreciate any insight into scaling proportions for a child. Specifically, regarding torso length and the back panel, I started with my own pack. My torso measures 21" from the C7 vertibrae to the hip shelf and I expected that to be the approximate measurement between the upper and lower attachment points of my shoulder straps. To my surprise, it measures 17". See the pic below:

So, my question is, since this measures approximately 80% of my torso length, should the same rule apply to the new pack? My son's torso measures 15.5", so the distance between upper and lower attachment points should be 12.5?

Obviously, I would want to build in some 'room to grow' but is my general logic sound?

Also, are there any other MYOG pack tutorials that you can recommend? Videos etc?

I would err on the side of too long a torso, rather than too short. But it is really hard to give any concrete answers without knowing more about the pack. for example, whether or not there will be a hip belt, how substantial it will be, will there be a framesheet of any sort etc. all affect how long a torso measurement I would want in a pack design.

I'll have to agree with Adie, it all depends overall intent of pack. I've only made two packs, by no means an expert, but the best learning experience was to really think long term what I will want to get out of the pack. By that I mean required volume, desired packload, what "seasonal kit" I'll be carrying. There is information out there, but it's difficult to find. I studied a lot of different designs, and others trial and error (thanks DaveC). The two that I made, one was 42L and the other was a 58L, both were pocketless but had daisy chains for modularity. The 42L would be my spring/summer/fall and the 58L would be my late fall/winter/packraft pack. By knowing your pack intent, I think you can search the specific questions you may have in designing (i.e. should straps thickness, width, etc.)
Like most people will tell you, it starts with a suspension system. Like Adie, I'd recommend possible 1-1.5" larger than actual torso, it allows growth, and as long as you use load lifters you should be able to get a pretty good fit on a child. I've made three different sets of shoulder straps, although they've turned out great, I often think that ordering a complete set would save at least 25% of overall construction time.
Start with backpanel and what frame system you'll use. I've used everything from 0.040" aluminum with 2.5" holes for weight purposes, to coroplast, to 3/8" sturdy foam with .75" wide x .125" thick aluminum bar stock for frame. They all work, you just have to find what works best for you. Quite honestly my favorite might be a full length thermarest prolite.
After frame, determine what you will need to confortable carry that. Do you need thick or thin foam pads? Wide? Narrow? Next think of lumbar support and hip belt, how necessary are they and what thickness?
Once you've got that figured out, you're done with the hard part. Now to the fun part, design of looks, volume, pockets.
Enjoy.

Cullen, I'm pretty sure that Golite is too short for you. The load lifters help you get away with it, but it is still sub-optimal.

Presuming that the pack you're making won't have lifters, I'd aim for effective torso length (middle of belt to center height of shoulder strap attachments) to be right around the measured torso length. This will be on the long side of ideal for moderate loads and thus allow for a bit of room for growth.

I have scaled down my light packs a couple times, at two different scales, for my kids at various ages. I strongly suggest you mock up the shoulder straps & hipbelt and sew them onto a piece of fabric so that you can test fit and then adjust to suit before sewing the final pack. A lot easier to adjust the mockup than the real pack.

1+ mocking up. For my first pack I mocked up the design using tyvek. The mockup lead to some adjustments. Then built a prototype out of 1.9 ripstop from the value bin. I took the prototype fully weighted for some test hikes. Those test hikes lead to me ripping out seams and adjusting things. The prototype rides great but still isn't perfect. Point being, test it out on the cheap fabric before you try it on the good stuff.

Regarding your original question about fit, I based my strap layout and length on the G4. I found that the shoulder strap placement-hip belt dimension (22") to be too large for me on the G4. What size Golite are you using?

So here it is. I followed general construction guidelines from Diyfunprojects.com, the G4, and numerous MYOG posts here, but used a few of my own design ideas as well ('bathtub' style bottom, padded back panel, roll top closure with side straps). I also followed the color suggestions of my 9 year old budding Denver Bronco fan! It may not be stealth, but at least I won't loose sight of him on the trail…and he loves it!

To call the sewing 'amateur' would be generous, and I made friends with my seam ripper as I worked through the sequencing, but I ended up with a fully functional pack for about $30 that weighs a scant 11oz. I estimate the volume of the main pack bag at 22L, and it is made of 200D coated oxford at 4oz/yd2.

A few questions:

The tops of the mesh pockets are sloppy. Any recommendations for a more polished look?
Any thoughts about the fit on my boy?

For the mesh pocket tops, a double layer of sil with a shock cord threaded inside would work. A could lines of well spaced stitching will ensure good retention between the fabrics. Put a small grommet on the inside at each end and secure the cord with knots. This way you can tighten it if/when the cord stretches.